An image forming apparatus is provided with a scanning optical device composed of a laser scanning unit (LSU) exposing laser light to a photosensitive drum to form an electrostatic latent image. The scanning optical device includes: a light source which emits laser light; a deflector which deflects the laser light; and optical components groups such as a lens or a mirror. The light source, the deflector, and the optical components groups are housed in an optical box which is mounted to a frame of the image forming apparatus.
The deflector has: a polygon mirror; a polygon motor which rotates and drives the polygon mirror; and the others. The optical box in which the deflector is housed is generally formed of a resin and is therefore easily thermally deformed by heating from the polygon mirror or the polygon motor rotating at a high speed. If the optical box may be thermally deformed, since positions of the optical components vary, a displacement in a main scanning direction and in a sub scanning direction occurs, causing degraded scanning precision.
Thus, in Patent Document 1, there is proposed a scanning optical device in which a sleeve provided on an extension line of a rotating shaft of a polygon mirror of a deflector is engaged with a hole formed in a frame of an image forming apparatus so as to thermally deform an optical box uniformly in a radiation direction around the sleeve. In this scanning optical device, even if the optical box is thermally deformed, since a relative positional relationship between the optical components is maintained, control to correct the amount of displacement is easily carried out, and intervals of color registration control against a color displacement can be extended.
Also, there is a problem that there may occur an image failure in which a fine striped pattern is printed, such as jitter or banding, due to rotary vibration exerted by a high speed rotation of the deflector (for example, 50,000 rpm).
As a countermeasure against such rotary vibration, in Patent Document 2, there is proposed a scanning optical device configured to add a counterweight to a rotating shaft of a polygon mirror and thus to displace a resonation point thereby to avoid resonation with an optical box. Also, in Patent Document 3, there is proposed a scanning optical device in which a stator yoke of a polygon motor is fixedly attached to a frame via a vibration suppression member. Further, in Patent Document 4, there is proposed a scanning optical device in which a deflector is disposed outside of a region surrounding a fixed point between an image forming apparatus and an optical box so as to easily attenuate the vibration.